It is known in the art to provide means for varying the compression ratio of a diesel engine in order to provide a relatively high compression ratio for cold starting and warm-up, where compression ignition is more difficult, and to provide reduced compression ratios for operating in other modes, particularly at high loads and speeds, to reduce peak combustion pressures and temperatures. Recently the emphasis for such arrangements is primarily to minimize emissions of nitrogen oxides (NOx) by operating at lower compression ratios where this is possible. Many devices have been proposed for compression ratio variation, including variable valve timing mechanisms and engine components such as pistons and cylinder heads with movable combustion chamber walls. In general these devices are relatively complex and add significant cost to the manufacture of an engine.
In spark ignition engines, dual-lobed cams with lobe selection mechanisms are known devices for varying valve timing, duration and lift thus changing valve timing. These devices normally provide for both advancing valve opening and retarding valve closing in order to obtain desirable performance characteristics. It is believed that dual-lobed cams with lobe selection mechanisms have not been utilized on diesel engines because the piston to cylinder head clearance is so small that altering intake and exhaust valve timing may result in contact of the pistons with the valves. A simple and relatively low cost apparatus and method for controlling compression ratio in a diesel engine is desired.